1. Field of the Invention
The present invention relates to a circuit for correcting a skin color of a human being. The present invention also relates to a method of correcting a skin color of a human being.
2. Description of the Background Art
A chrominance demodulation circuit of a television receiver comprises a circuit for correcting a skin color of a human being, i.e., flesh correction circuit. Hues of a reproduced image picture differ between different broadcasting stations, between different programs provided by the same broadcasting station, and between broadcasting and reproduction on a VCR. Distortion in a transmission system including a cable also causes the hues to alter. The chrominance demodulation circuit automatically rectifies such deviation in hues on the basis of a flesh color.
Correction of a skin color of a human being (flesh color correction) will be described while referring to FIGS. 2 and 3. FIG. 2 shows how a phase difference between a chrominance signal and a demodulated reference signal relates to a reproduced color. The direction of an axis +I corresponds to a phase of a flesh color. If the chrominance signal has a large phase difference with the demodulated reference signal whose phase is in the direction of the axis +I, the chrominance signal is flesh-color corrected by reducing the phase difference, i.e., by phase-shifting the chrominance signal in the direction of an arced arrow toward the axis +I (phase correction). When a color to be demodulated is nearly a flesh color, only a small amount of the phase correction is necessary.
FIG. 3 shows how the phase difference between the chrominance signal and the demodulated reference signal is corrected. A subcarrier signal S as preliminarily phase-shifted is used as the demodulated reference signal whose phase is in the direction of the axis +I. In the description hereinafter, the subcarrier signal S and the demodulated reference signal will be regarded as the same unless identified as different. The subcarrier signal S is shown as a vector OA and a chrominance signal C is shown as a vector AB. In chrominance demodulation which does not require phase correction, a phase difference between the signals S and C is equal to an angle .alpha..
A reference signal RS is derived in correspondence to the phase difference .alpha. between the signals S and C used for chrominance demodulation, a phase difference .beta. between the reference signal RS and the chrominance signal C being smaller than the phase difference .alpha.. Hence, demodulation of the chrominance signal C using the reference signal RS is equivalent to phase correction of the chrominance signal C toward the axis +I. In other words, using a vector OB as the reference signal RS allows the phase difference .beta. between the reference signal RS and the chrominance signal C to become smaller than the phase difference .alpha..
FIG. 1 is a block diagram of a flesh correction circuit which is described in "A chrominance demodulator IC with dynamic flesh correction," IEEE Transaction on consumer electronics, 1976, pgs. 111-117. A chrominance signal C is received through a terminal 1 by a chrominance amplifier 5 where it is amplified, and given to a phase detector 10. Through a terminal 2, a subcarrier signal S is allowed to a hue adjusting circuit 6, and thence to the phase detector 10.
The chrominance signal C is given to a phase shifter 12 for phase-shifting a subcarrier signal, too, through a limiting amplifier 11. The phase shifter 12 also receives an output from the phase detector 10. An output from the phase shifter 12 is given to an adder 13 where it is added to the subcarrier signal S which has been given to the adder 13 through another limiting amplifier 11. As a result, a reference signal RS is generated. The reference signal RS is supplied to an I-axis demodulation circuit 7 without being phase-shifted and, via a 90-degree phase shifter 9, to a Q-axis demodulation circuit 8.
The I-axis demodulation circuit 7 and the Q-axis demodulation circuit 8 also receive the chrominance signal C and demodulate the same. The I-axis demodulation circuit 7 outputs an I-axis demodulated signal to a terminal 3. Likewise, the Q-axis demodulation circuit 8 outputs an Q-axis demodulated signal to a terminal 4.
The phase detector 10 multiplies the subcarrier signal S by the chrominance signal C. The subcarrier signal S is shown in FIG. 4A and the chrominance signal C is shown in FIG. 4B. As can be seen in FIGS. 4A and 4B, the chrominance signal C has a phase delay of 45 degrees with respect to the subcarrier signal S.
The phase detector 10 generates a signal as shown in FIG. 4C. The signal is supplied to the subcarrier phase shifter 12 where it is processed in such a manner that a potential smaller than a predetermined offset bias is eliminated. The signal thus processed gates the chrominance signal C supplied to the subcarrier phase shifter 12; that is, the subcarrier phase shifter 12 demodulates the chrominance signal C into a signal as shown in FIG. 4D and outputs the same to the adder 13.
The adder 13 adds the demodulated chrominance signal C to the subcarrier signal S, thereby the reference signal RS being derived which corresponds to the vector OB of FIG. 3. The larger the potential of the output signal from the subcarrier phase shifter 12 is, the larger a phase difference between the reference signal RS and the subcarrier signal S becomes. In other words, the smaller the potential the offset bias is, the less desirable the effect would be of the flesh correction where a phase difference between the subcarrier signal S and the chrominance signal C is large. The flesh correction should be limited to where the phase difference between the subcarrier signal S and the chrominance signal C is small to a certain extent. If all hues are skin-color corrected without exception, a reproduced image will have an unnatural tone.
Thus, the conventional flesh correction circuit includes a complicated system for detecting a phase difference between the subcarrier signal S and the chrominance signal C and generating the reference signal, and therefore, requires a large size.